The invention relates in general to hybrid vehicle systems, and in particular to power control of energy storage in hybrid electric drives and vehicle systems. Hybrid systems are advantageous due to their ability to increase the fuel efficiency of the vehicle system and to reduce air pollution.
Typically, hybrid vehicle systems include a low side power source and a high side power source. The low side power source and the high side power source operate in conjunction to supply power to a device such as a motor, which may be used, for example, to propel a hybrid vehicle. The low side power source, which typically has a high energy density, may comprise a battery, a heat engine, an ultracapacitor, a flywheel or the like. The heat engine may include any engine that burns a fuel to produce mechanical work, such as an internal combustion engine, a turbine engine, a diesel engine or the like.
Some vehicles use electric traction motors as a component of the high side power source. Electric traction motors are typically connected to a link, such as a power bus to deliver power to the motor. One or more on-board alternators may be used to provide the power to the link. Under certain operating conditions, such as when the vehicle is decelerating or is maintaining speed on a downhill grade, the back electromotive force (“EMF”) produced by the electric motors is greater than the nominal voltage of the power bus dc link. Under such conditions, the electric traction motor may cease acting as a motor and become a generator. This process, known as dynamic braking may be used to reduce wear on the mechanical brake system components of a vehicle. In the case where the vehicle is a locomotive, dynamic braking may reduce brake wear on the locomotive and also on all of the rail cars of the train. A grid resistor may be used to dissipate the electric power as heat produced by the electric motor during dynamic braking. Additionally, systems have been developed to recover some of the energy that is typically wasted as heat during dynamic braking. The recovery of this wasted energy is known as regenerative braking.
In series hybrid vehicle systems, the power source and associated controls may be operated in an “on/off” mode, where the energy source when it is “on” operates at a predetermined speed or possibly over a narrow speed range to recharge the high side power source, which may comprise a traction battery. The traction battery is typically operated in a manner that allows for either periodic discharge during vehicle acceleration, or recharge during periods of regenerative braking when the vehicle is decelerated or maintaining speed. In other words, one method of controlling the traction battery is to operate the low side power source to maintain the state of charge (SOC) of the traction battery within a given range. However, the transient response of such systems is reduced when a sudden high power vehicle maneuver is required during the period when the power source is in the “off” mode.
There is, accordingly, a need to provide an improved method and system to control the operation of the low side power source and the high side power source in a hybrid vehicle.